Depending on the application, radio frequency (RF) amplifiers can be classified as low noise, low power receiver amplifiers, or as high power transmitter amplifiers. The receiver amplifier is designed to process weak signals. For high power transmitter amplifiers, noise is of minor interest since the transmitted signal is usually known and its amplitude can be controlled. What is more important for high power transmitter amplifiers is efficiency. To obtain high efficiency, power amplifiers are operated near saturation. Unfortunately, when operated at saturation, the output of any amplifier is distorted because the gain decreases from what it is for inputs of smaller amplitude. In fact, at saturation--when the input to be amplified is of large amplitude--both the amplitude and phase of the output signal are distorted. The output at saturation is not amplified as much as for smaller amplitude inputs, and there is a slightly longer delay before the amplified signal is produced, as compared to the delay for smaller amplitude inputs; i.e. the output is phase-shifted. Both of these two effects distort the signal, and this distortion is especially of concern in telecommunications.
In the current US cellular band, there are two 25 MHz frequency bands allocated for cellular phones: the frequency band 824 MHz to 849 MHz is for mobile transmission and the 869 MHz to 894 MHz band is for base station transmission. Each of these 25 MHz bands is split into 832 channels, which are each 30 kHz wide. An amplifier in a cellular system can amplify a single channel, in which case it is called a narrow-band amplifier, or the entire 25 MHz band of 832 channels, in which case it is called a wideband amplifier. Depending on the architecture, the amplifier may shift the 25 MHz spectrum from the high frequency band (869-894 MHz) to the 0-25 MHz frequency band--called the baseband--and then process the signal.
If the input to an amplifier drives it to saturation so that it behaves non-linearly, several undesirable consequences result. First, the signal being amplified is distorted. Next, if the amplifier is a narrow-band amplifier, signals from amplifiers for adjacent channels will be affected. And finally, if the amplifier is a wideband amplifier, not only will the signal on each channel be distorted, but its distortion will add to the distortion of other signals it is amplifying, on other channels. (This last effect is called intermodulation distortion.) It is therefore of great interest, in some important applications such as cellular communications, to keep the output of an amplifier as near to a scaled up version of the input as possible, i.e. a linear function of the input.
The simplest way of ensuring this linear behavior is to operate the amplifier far below saturation. The disadvantage is lower power efficiency due to the relatively high quiescent bias voltage of such an amplifier as compared to the voltage for an amplifier operating at saturation. In general, if a power amplifier is operated at a power level where it is linear, it is power inefficient; if it is power efficient, it is normally not linear. The challenge is to maintain, simultaneously, linearity and power efficiency.
Several approaches to meeting this challenge have already been developed. All of these approaches operate an amplifier near saturation, but compensate for the non-linearity that results in different ways. Some of these approaches are analog and some are digital. The analog methods are fast, and so suitable for use in wideband amplifier systems, but are not as accurate as the digital methods. The digital methods, as they have been implemented so far, are too slow for use in a wideband amplifier system, but are more accurate and can be made to adapt to slow changes in an amplifier's nonlinearity due to aging, temperature changes, and other factors. The single lament over digital methods is that, as implemented so far, these methods are too slow to be used for more than a few 10's of kHz of signal bandwidth.
Among the various methods of correcting for an amplifier's non-linearity when it operates near saturation, some directly manipulate the high frequency RF carrier signal. To work with signals at these high frequencies, analog methods must be used. Feedforward correction and RF predistortion, both analog methods, linearize amplifiers this way.
Other techniques work with the baseband signal, then up-convert it to the RF carrier frequency. Since the signal corrected in these methods--the baseband signal--is at a lower frequency, digital methods can be used, but only for a fraction of the baseband signal, at least as these methods have been implemented so far. Cartesian feedback--a digital method--and digital predistortion are two methods that apply corrections to the baseband signal.
The Cartesian feedback method was designed initially for transmitter amplifiers where the baseband signal is available; it adjusts the baseband signal according to feedback from the output of the amplifier, then up-converts the signal to the RF carrier frequency for transmission. It makes use of a quadrature representation of the signal being amplified. In such a representation, a signal X(t) modulated by a carrier frequency .omega..sub.c is expressed in terms of orthogonal basis functions cos(.omega..sub.c t) and sin(.omega..sub.c t) as EQU X(t)=I(t) cos(.omega..sub.c t)+Q(t) sin(.omega..sub.c t)
where the information being communicated is conveyed by the time-dependent coefficients I(t) and Q(t). These coefficients, called the quadrature components, can be asserted to represent rectangular components of a two-dimensional vector. This vector can also be expressed in polar coordinates (.rho., .theta.) where ##EQU1## In Cartesian feedback, the baseband input signal is decomposed into quadrature components, I and Q. A portion of the output signal is demodulated and also decomposed into quadrature components, I.sub.b and Q.sub.b, to provide feedback. Then I.sub.b and Q.sub.b are subtracted from the original I and Q input drive signals to generate a loop error signal.
Another way to linearize the amplifier output is by predistorting the input signal prior to amplifying it, making the overall system appear linear. In this method, a so-called predistorter is placed between the input signal and the power amplifier. Most of these predistorter methods can be classified as either RF analog predistortion or baseband digital predistortion.
The RF analog predistorter places a network of active and passive components designed to compensate for the amplifier characteristics between the input signal and the amplifier. Diodes are often used in the network to provide the nonlinear compensation. The correction can be fixed--meaning that once the network is implemented the correction will not change--or adaptive, in the sense that the correction is continually adjusted as the amplifier characteristics change.
RF analog predistortion is suitable for wideband application. Due to the difficulty of designing a circuit that can match the amplifier perfectly, however, the linearization is not as complete as can be achieved using digital signal processing. But to use digital signal processing, the signal must be down-converted to the baseband because digital signal processing equipment today is not fast enough to accurately digitize signals at frequencies in the 10's of MHz.
So digital predistortion first down-converts the signal to be predistorted, applies a predistortion using digital signal processing, and then up-converts the signal for the amplifier. In fixed baseband digital predistortion, the predistortion is not adjusted to account for changes in the non-linearity of the amplifier. In adaptive baseband digital predistortion, the predistortion is continually adjusted based on an error signal determined by comparing feedback from the amplifier with feedforward from the original input signal. To determine how to adjust the predisortion using digital signal processing is a time-consuming process, compared to how long it takes to apply a predetermined predistortion. Therefore, feedback in the baseband digital predistorter is used only for compensating for slow drift of the amplifier, caused by changing temperatures, aging and other factors; it is not used for real-time adjustment of the predistortion values.
There are now several adaptive baseband digital predistortion methods. In one of the most general and powerful methods, the input signal X is represented in quadrature format. The quadrature signals, I and Q, are used as indices into a two-dimensional look-up table holding the predistortion values needed to generate a predistorted signal, X.sub.d. The predistorted input signal X.sub.d is also implemented in quadrature format, I.sub.d and Q.sub.d. With this method, since I and Q, in combination, carry both amplitude and phase information, the amplifier's nonlinear amplitude and phase characteristics can both be compensated for by using the predistorted input X.sub.d.
The number of quantization bits--the resolution of the digital signal--plays an important role in the performance of the system. The more bits, the more precisely the signal is represented, and the better the cancellation of the amplifier nonlinearity. But more bits require a larger look-up table. It is known that 10 quantization bits for each of the two quadrature signals, I and Q, are needed to achieve acceptable nonlinearity correction. For this resolution, the size of the two-dimensional look-up table is 20M bits (2.sup.20 .cndot.20 bits). A predistorter based on this method can be made adaptive, but the adaptation is slower than in other baseband digital predistortion methods because the look-up table is larger.
The other predistorter methods use one or more one-dimensional tables to reduce the look-up time. This is possible because both an amplifier's amplitude and phase distortion depend only on the amplitude of the input signal. Therefore, if the amplitude of the input signal, .rho., is extracted from the I-Q representation, then it can be used as an index into both a table for phase correction and another table for amplitude correction, or as an index into one table with complex number corrections providing simultaneously both an amplitude and a phase correction.
The approach described in the preferred embodiment of the present invention uses two one-dimensional look-up tables. The two one-dimensional tables, combined, are much smaller than a single two-dimensional look-up table, so updates to the tables can be applied much faster. This approach, however, does require converting from a rectangular (I and Q) to a polar representation (.rho. and .theta.) of the input to be amplified.
All of these methods of digital baseband predistortion offer more freedom in handling cancellation of nonlinearity than do analog methods, since the cancellation is performed through software. The disadvantage, as these methods have been implemented, is small bandwidth.
Because of the limitation in digital signal processing speed, it is not possible to implement a digital predistorter with a 20 MHz bandwidth using digital signal processing in a conventional, sequential digital signal processing architecture, as in the prior art. For example, suppose a digital predistorter uses a digital signal processor with a 50 MHz clock rate and 12.5 MHz instruction rate and a look-up table. Between every two data samples, the digital signal processor must read data from the look-up table and digitally process it, i.e. perform operations of addition, subtraction or multiplication. Assuming ten instructions are required to process one digitized sample of the input, the fastest data sample rate is 12.5 MHz/10=1.25 MHz. According to the Nyquist sampling theorem, to avoid aliasing, the sample rate must be at least twice the maximum frequency component of the real-time spectrum. With a sample rate of 1.25 MHz, the actual linearized bandwidth is thus limited to 0.625 MHz maximum. Therefore, a sequential digital signal processing implementation using contemporary hardware cannot be used in a wideband amplifier system.
The prior art of digital predistortion is thus all implemented in only narrowband amplifier systems. These digital predistorters all work well for a single channel (10-30 kHz) or a small number of channels in the mobile communication band. When the bandwidth increases to several MHz, however, they do not work fast enough.